1. Field of the Invention
The present invention relates to a plugged honeycomb structure manufacturing method, and a plugged honeycomb structure. More particularly, the present invention relates to a plugged honeycomb structure manufacturing method capable of easily manufacturing a plugged honeycomb structure which can avoid a phenomenon of rapid increase of pressure losses generated by cell openings clogged with particulate matters on an inlet side, and a plugged honeycomb structure capable of reducing pressure losses and effectively preventing clogging with a solid content which does not disappear during burning of ash, iron oxide or the like.
2. Description of the Related Art
A large amount of particulate matters mainly containing soot (carbon black smoke) are contained in an exhaust gas discharged from a diesel engine, a gasoline lean burn engine, or a gasoline direct jet engine. Since the particulate matters discharged to atmospheric air cause environmental pollution, an exhaust gas system of the diesel engine is provided with a honeycomb filter for trapping the particulate matters.
As the honeycomb filter for use in such purpose, a plugged honeycomb structure is used in which one end portion of a part of cells is plugged in one end of a honeycomb structure having a plurality of cells partitioned by porous partition walls and extending through the structure in an axial direction, and one end portion of each of remaining cells is plugged in the other end (see, e.g., Japanese Patent Application Laid-Open No. 2001-269585).
A treated gas (exhaust gas) flows into the cell whose inlet-side end is not plugged and whose outlet-side end is plugged in this plugged honeycomb structure, passes through the porous partition walls, moves into the cell whose inlet-side end is plugged and whose outlet-side end is not plugged, and is discharged from this cell. In this case, the partition wall constitutes a filtering layer, and particulate matters such as soot in the gas are trapped by the partition walls and deposited on the partition walls.
Moreover, as the honeycomb structure for trapping the particulate matters, a filter is also disclosed in which one end portion of a part of the cells is sealed in an only one end of the above-described honeycomb structure (see, e.g., Japanese Patent Application Laid-Open No. 2004-251137). When this honeycomb structure is manufactured, in order to eliminate a dimensional difference during firing, cell end portions are first filled with a material (plugging material) constituted of a ceramic or the like as a plugging portion in not one end, but opposite ends of the honeycomb structure, and the structure is fired so that a contraction amount during the firing becomes appropriately equal in opposite ends. After the firing, the plugging portions of the cells in one end are removed to thereby obtain the honeycomb structure. Alternatively, it has been proposed that a honeycomb formed body having a length which is not less than twice a necessary length be obtained, and plugged in the same manner as in the above-described method. After the formed body is further fired, the resultant fired article is cut in the center to thereby obtain two halves.
However, in the plugged honeycomb structure described in Japanese Patent Application Laid-Open No. 2001-269585, since the openings of all the cells on one end side are plugged, there is a problem that there are many pressure losses.
Moreover, when a diesel particulate filter (DPF) for use in purifying the exhaust gas of the diesel engine is used, the trapped particulate matters are gradually deposited in the filter. When they are left to stand, a filtering performance degrades. Therefore, when the particulate matters are deposited to a certain degree, the filter is heated or treated otherwise to burn and remove the deposited particulate matters, and a filter function is regenerated. However, in a case where the conventional plugged honeycomb structure is used, there has been a problem that a solid content which does not disappear by the burning of ash, iron oxide or the like is gradually deposited, and the structure is easily clogged. Furthermore, in the inlet-side end, there has been a problem that the particulate matters stick to a portion provided with a plugging member, the particulate matters are gradually deposited from the portion as a starting point to plug the openings of the cells on an inlet side, and therefore the pressure losses of the plugged honeycomb structure rapidly increase.
Moreover, in the honeycomb filter in which one end portion of each of a part of the cells is plugged in the only one end of the honeycomb structure, there is used a honeycomb structure which has heretofore been used. Since such honeycomb structure generally has such a shape that the pressure losses are reduced by enlarging a filtering area of the porous partition wall, a ratio (length/outer diameter) of a length of the honeycomb structure in an axial direction to an outer diameter of the honeycomb structure is 0.9 or more. Therefore, there has been a problem that the length of the resultant honeycomb filter in the axial direction comparatively increases, and an installation space is wasted.
Moreover, as a method of manufacturing such honeycomb filter, there is disclosed a method of removing the plugging portions of the cells in one end after the firing. However, in such manufacturing method, there is a problem that the removed plugging portions in one end are wasted, and costs increase. In a case where the honeycomb filter is manufactured by a method of cutting the fired article in the center to obtain two halves, it is necessary to form, dry, plug, and then fire the honeycomb structure having a length which is not less than twice the length of the final honeycomb structure. Specifically, for example, when a dimension of a final product indicates a diameter of 191 mm and a length of 203 mm as one dimension of a standard honeycomb structure, an elongated article having a diameter of 191 mm and a length of 406 mm is obtained. When the elongated article is fired in a state in which the opposite ends are plugged, it becomes very difficult to discharge a forming binder from the honeycomb formed body, and therefore a sophisticated firing control technology is required. Since a firing time is set to be very long, there is also a problem that a firing capability degrades, and the costs increase.